Most of the intelligence needed to handle a telephone call originating from a public telephone, e.g., a public payphone, has been moved from the central office to the public telephone. This move has made it easier to initiate telephone service without requiring certain signaling, e.g., coin control signals, and/or particular connections, e.g., a connection to a telephone operator. Because of this move, certain individuals have discovered that they can connect ("clip-on") their own telephone sets directly to the telephone line connecting to a public telephone, thereby bypassing the public telephone, and thus make a "free" telephone call. Clearly, such an action is fraudulent and amounts to a theft of telephone service.
One prior solution uses an ISDN based public telephone to deal with this type of fraud. It can be appreciated that a solution that employs ISDN is indeed expensive. Another prior solution arranges a public telephone so that it continuously transmits a so-called out-of-band signal, e.g., a 16 kHz signal, that is expected by the local central office to prevent "clip-on" fraud. That is, the central office will drop calls seemingly originating from a public telephone if the central office fails to receive the out-of-band signal. The reasoning behind this solution is that a fraudulent non-public telephone would not be arranged to transmit such a signal. It is apparent that this arrangement may be easily avoided simply by adding an oscillator capable of generating the expected out-of-band signal to the fraudulent non-public telephone.